The invention relates to a transparent glass ceramic containing .beta.-quartz crystalline solid solutions, which ceramic is colorless and has in the temperature range from 20.degree. - 300.degree.C a thermal longitudinal expansion of less than 15 .times. 10.sup..sup.-7 /.degree. C.
It is known that fine-grain crystalline solid solutions having an .beta.-quartz or .beta.-spodumene structure can be precipitated by heat treatment from glasses containing essentially the components SiO.sub.2, Al.sub.2 O.sub.3, P.sub.2 O.sub.5, Li.sub.2 O, MgO and ZnO, if the glasses contain small quantities of so-called nucleating agents as well as the said main constituents. The fine-grain and uniform precipitation of these types of crystals has the effect that the formed glass-crystal mixed body, which is generally designated as glass ceramic, is distinguished as regards most properties in a characteristic manner from the glass from which it was formed. Particularly outstanding is the unusually low thermal expansion of such bodies, which can even be negative, and which is determined by special properties of the crystal structure of the said crystalline solid solutions. The low thermal expansion leads to an extreme stability with respect to change in temperature of such glass ceramics, and this is a property which is extremely significant for many uses.
Normally, the glass ceramics are opaque or have a porcelain-like translucency. Under certain conditions, for example, when the formed crystals are much smaller than the wavelengths of the visible light, or when the refractive indices of the crystals and of the residual glass show a sufficiently small difference from one another, glass ceramics can also be transparent. Suitable glass compositions and processes for the heat treatment in order to produce transparent glass ceramics with a very small thermal expansion are for example indicated in German Offenlegungsschrift No. 1 596 860. According to this teaching, the initial glass has to contain 1.5 to 6% by weight of TiO.sub.2 and 0.5 to 3% by weight of ZrO.sub.2 as nucleus formers, so that a transparent glass ceramic can be produced. The content of TiO.sub.2 has the effect, as a displeasing secondary phenomenon, of imparting to the glass ceramic a distinct brown coloring. This is all the more pronounced if, as instructed in U.S. Pat. No. 3,157,522, only TiO.sub.2 is used as nucleus former and in fact in an amount of 3 to 7% by weight.
This brown coloring is normally undesirable and reduces the value of the product. Accordingly, experiments have been undertaken for developing colorless glass ceramics with small thermal expansion. One possibility of obtaining such bodies is described in German Offenlegungsschrift 1 496 098. The glass ceramics indicated therein do not contain any TiO.sub.2, but only ZrO.sub.2, as nucleus-forming constituent, and this in an amount of 2 to 15% by weight. Since the initial glasses being used contain at least 60% of SiO.sub.2 and also other predominantly high-melting constituents, fusion temperatures of at least 1600.degree.C and sometimes up to 1800.degree.C are necessary for the preparation thereof and these temperatures can only be utilized at very great expense. This circumstance greatly restricts the utility of the said glass-crystal mixed bodies, although they have the required freedom from color and in part are satisfactory as regards their thermal expansion.